1. Field of the Invention
The present invention relates to an electronic sewing machine and more particularly to a machine having an improved electronic control for the stitch forming members, namely for the transverse oscillations of the needle bar and for the longitudinal displacement of the member feeding the fabric on which the said stitch is produced.
2. Description of the Prior Art
In the present art electronic sewing machines are known which provide the drive and control of the transverse oscillations of the needle bar and the longitudinal displacement of the cloth conveyor member via corresponding actuators which are moved in accordance with given moments of synchronism and given positioning times. The household sewing machines of this type which are known to applicants generally memorize information relative to the positions of the needle bar and the feed member in accordance with a digital code in a read-only memory or in another static memory, such as disclosed in U.S. Pat. No. 3,984,745. Such memory is merely a data bank which is explored sequentially in conventional manner during the performance of a predetermined sewing program, starting with the first stitch of a selected pattern, up to the last stitch of said pattern, and thus repetitively. Sewing machines of this type, however, have the drawback of not permitting a processing of the data memorized for such patterns and therefore they do not possess operational flexibility in the use of such data, so that it is not possible to change the execution of the memorized patterns in order to vary any of their characteristic parameters such as width, density of stitch, alternative combinations, etc.
Several of the problems which have existed in the prior art due to various limitations resulting from the design of such electronic sewing machines have been overcome by the system shown and described in commonly owned U.S. patent application Ser. No. 973,386, filed on Dec. 26, 1978, and entitled "Improved Household Type Sewing Machine Having Microprocessor Control". The contents of this commonly owned application, in which it is possible to vary substantially, as the operator desires, the actual execution of the various patterns memorized are specifically incorporated by reference herein in their entirety. Thus, the problems existent in the prior art known to applicants up to the filing of that application are discussed therein; such as with respect to U.S. Pat. Nos. 4,108,093; 4,086,862 and 4,116,144 and need not be reiterated herein.
However, although the electronic sewing machine described in U.S. patent application Ser. No. 973,386 is a satisfactory solution which overcomes many problems previously existent in the prior art, it is essentially an analog solution having certain possible drawbacks with respect to stability, reliability, testability and maintainability. These disadvantages are overcome by the improved digital type solution of the present invention which provides a faster response at the same power levels with the same actuator than is possible with the analog solution of U.S. patent application Ser. No. 973,386. Suffice it to say in describing the system of U.S. patent application Ser. No. 973,386, that the DC actuators of the stitch forming members, namely the needle bar and the feed member, are controlled through an analog type servo loop and the digital position commands provided from the microcomputer or microprocessor for arriving at the desired position of the actuators. In accomplishing this, the digital command signals to the actuators are converted into corresponding analog signals by a suitable digital-to-analog converter placed downstream of the microcomputer. The position control in the aforementioned analog type servo loop is therefore accomplished by operating on such analog signals since the analog position transducer, which is generally a potentiometer, generates a voltage proportional to the actual position of the stitch forming controlled member which is fed back and compared, in analog fashion, with the control voltage signal to determine the position error, if any. The resultant voltage, which is proportional to the position error, through a suitable power amplifier, controls the movement of the corresponding actuator, for bight or feed, in such sense and direction so as to reduce the position error to zero. In this position control servo loop an internal speed servo loop is inserted for intrinsic dynamic reasons (the positioning times required being on the order of 15 to 20 milliseconds and, therefore, being comparable to the time constants of the actuators). The speed signal which corresponds to the actual speed of movement of the actuators is not, however, generated by a separate transducer for obvious reasons of cost, but is rather obtained as a variation in the time of the positioning signal. Therefore, in order to obtain this speed signal, a voltage proportional to the derivative of the position and therefore to the speed is generated. The previously mentioned position error rather than being sent directly to the power drive of the actuators is therefore compared with this speed signal and the resultant comparison signal, in its turn, drives the power amplifiers of the actuators. Accordingly, this analog embodiment of the control system described in U.S. patent application Ser. No. 973,386 comprises two analog servo loops, one within the other, with one relating to the position and the other to the speed of the actuators. These two analog servo loops make it possible to solve the problem of the positioning of the electronic sewing machine feed or bight actuators in the desired dynamic terms. This analog solution, which is relatively satisfactory in view of what has been stated above, does not, however, represent an optimum solution such as, for example, providing a slower response at the same power levels with the same actuator than is obtained by the improved system of the present invention. Thus, despite digital servo control of positioning of machine tools being well known, such as disclosed in U.S. Pat. Nos. 3,504,362; 3,483,362; 3,795,851; 3,546,599; 4,090,116; 2,885,613; 2,932,471; 3,063,311; 3,644,897; 3,958,109; 3,752,098; 3,189,805; 3,225,279; 3,643,076; 3,758,757; 3,226,617; 3,065,394; 3,939,390 and 3,872,808, there are no prior art systems known to applicants which have the overall capabilities of the present invention for providing a flexible and efficient electronic sewing machine.